Drilling rigs used for offshore drilling generally fall into two categories: gravity based drilling rigs and floating drilling rigs. Free-standing drilling rigs, such as fixed platform or compliant tower drilling platforms, include legs directly anchored to the seabed. Floating rigs, such as spar rigs, semi-submersible rigs, floating production systems, or drillships may be anchored to the seabed by guide cables or the like, but are otherwise free to float by buoyant forces at the surface. As the floating platform moves in response to surface conditions, the drill string or casing being lowered by the platform must be held with relatively good vertical stability with respect to the ocean floor.
In some drilling platforms, a passive heave compensation system may be utilized to prevent the drill string or casing from moving vertically during rig operations. In some embodiments, a passive heave compensation system may include one or more pistons positioned at the travelling block to allow the drill string to move upward or downward relative to the drilling platform as the drilling platform is affected by surface conditions. In other drilling platforms, an active heave compensation system may be utilized. In some such systems, a drawworks may be used to actively raise or lower the drill string in response to movement of the drilling platform.
A drawworks is a common piece of oil field equipment that is used in oil and gas drilling and production. A drawworks is positioned to lift and lower a travelling block in a drilling rig. The travelling block is suspended by a wireline from the derrick of the drilling rig, and is typically used to raise and lower drill string and casing out of and into a wellbore. Often, the travelling block couples to the drill string or casing via a top drive. The top drive is a motor used to rotate the drill string within the wellbore during a drilling operation. In an active heave compensation system, the drawworks must raise and lower the travelling block relative to the level of the drilling platform as the drilling platform moves upward or downward in response to the surface conditions. The drawworks must continuously operate by raising or lowering the travelling block during the entire active heave compensation operation.
Because a typical drawworks generally includes an AC induction motor coupled to the spool of the wireline by a gearbox, a large amount of energy is wasted during active heave compensation in simply causing the drawworks to rotate, stop, and rotate in the other direction due to the large amount of inertia of the rotor of the AC motor and the inherent losses of the gearbox.